4. Choosing Among Alternatives

A few years after people move into the first colony, the system should
settle down and operate as described in chapter 1. But why is the colony
shaped as a torus and located at L5 with ore supplies from the Moon? Why
is it not a sphere out at the asteroids or near a moon of Mars, or a
cylinder in geosynchronous orbit around the Earth, or some other
combination of alternatives? What are these alternatives, and why were
they rejected? The purpose of this chapter is to answer these questions
by evaluating reasonable alternatives in terms of the goals of the design
study (ch. 1) and the criteria laid out in
chapters 2 and 3.

A successful systems design combines subsystems satisfying various
conflicting criteria to produce a unified working entity. The parts of
the space colony- transportation, mining, the habitat, manufacturing,
agriculture, and so on - must interact and interrelate in such a way that
the demands of each for energy, raw materials, manpower, transport, and
waste removal can be met by the overall system. In turn this system must
satisfy the physiological, cultural, architectural, and physical criteria
necessary to maintain a permanent human community in space using
near-term technology and at a minimum cost.

In 10 weeks the study group was able to assemble only one reasonably
consistent picture of life in space; there was no time to go back through
the system and attempt to find optimal combinations of the subsystems.
Moreover, again because time was short, many of the comparisons among
alternative subsystems were more qualitative than study group members
would have liked.

Effort devoted to alternatives depended upon the particular subject. A
great deal of time was spent considering different forms for the habitat,
how to handle the shielding and how to process lunar material. Less time
was given to considering alternative patterns of siting the colony and
its parts, of different ways to achieve life support, or of various
possible transportation systems. In some cases much effort was expended
but few alternatives were generated; an example is the system for moving
large amounts of matter cheaply from the Moon to the colony. No
alternative at all was found to the manufacture of solar satellite power
plants as the major commercial enterprise of the colony.

It is important to realize that the alternatives described in this
chapter constitute a major resource for improving the proposed design and
for constructing new designs that meet other criteria. Rejection of any
concept for the current "baseline system" does not mean that concept is
fundamentally flawed. Some alternatives were rejected because they failed
to meet the criteria, which were deliberately chosen conservatively and
might well be changed on the basis of future experience or under
different assumptions. Others were rejected simply because information
about them was incomplete. Yet others were not chosen because their
virtues were recognized too late in the study to incorporate them into a
unified overall picture.

The alternatives might also be useful for designing systems with other
goals than permanent human settlement in space; for example, space
factories with temporary crews, or laboratories in space. Alternatively,
new knowledge or advances in technology, such as the advent of laser
propulsion or active shielding against ionizing radiation, might make
rejected subsystems very desirable.

What shape is most suitable to house this colony of 10,000 people? The
question is particularly interesting for several reasons. The appearance
and arrangement of the habitat are most obvious and understandable by
everyone, being the most direct exhibition of the reality of the idea of
the colony - seeing the form is believing - and the habitat natutrally
attracts a great deal of attention although it is only one part of a much
larger system. Moreover, the reader may already be aware of one or more
possibilities: the rotating cylinders proposed by O'Neill
(ref. 1), the
torus of Von Braun (ref. 2), and their corresponding entities in the
science fiction of Arthur C. Clarke (refs. 3, 4). The subject is also one
particularly suited for systematic treatment and can serve as an
excellent example of the methodology of systems design.

Some General Considerations

Because it is expedient, although not entirely justified, to treat the
shielding which protects against the dangerous radiations of space
separately from the choice of the geometry of the habitat's structure,
that problem is left to a subsequent section. Subject to possible effects
of the shielding, the choice of habitat geometry is determined by meeting
the criteria of the previous chapter at minimum cost. In considering how
different configurations may supply enough living space (670,000 m^2) and
how they meet the physiological and psychological needs of people in
space, the following discussion uses the properties of materials outlined
in
(refs. 5, 6). The possible "smooth" shapes are the ones generated
from the curves in
figure 4-1.

Four fundamental configurations arise:

A sphere-by rotating curve 1 about either axis

A cylinder-by rotating curve 2 about the z axis

A torus-by rotating curve 3 about the r axis

A dumbbellby rotating curve 3 about the z axis

Neglecting secondary effects from variations in pseudogravity and
localized bending stresses from discontinuities in deformations, the
study group concluded that all possible membrane shapes, that is, any
possible habitat, must be one of the four simple forms described above or
some composite of them as shown in figure 4-2.

The desire to keep structural mass small favors small radii of
curvature. As figure 4-3 shows, the wall thickness to contain a given
pressure drops quickly with decreasing. Of course, structural mass can
also be reduced by lowering the pressure of the gas. Both possibilities
turn out to be useful.

A Rotating System With 1 g at Less Than 1 rpm

Rotation is the only feasible way to provide artificial gravity in
space. Pseudogravity depends upon both rotation
rate and radius of rotation, and figure 4-4 shows the lines of
constant pseudogravity as functions of these two variables
(ref. 7). On
the graph are shown a number of rotating systems: C-l through C4 are the
rotating cylinders proposed earlier (ref. l) by O'Neill; T-1 is a torus
and S-1 is a sphere described later in this chapter; Arthur C. Clarke's
Rama (ref. 4) is shown, as are space stations of Gray (The Vivarium)
(ref. 8), Von Braun
(ref. 2)
, and Tsiolkovsky (ref. 9). Obviously only
systems with radii of rotation greater than 895 m can lie on the line g=
1 below 1 rpm.

An aluminum cylinder like C-3 would weigh about 42,300 kt and have a
projected area of 55 X I0^6 m^2, enough to hold 800,000 people - rather than
the 10,000 people of the design criteria. Similarly a sphere of radius
895 m would hold 75,000 people and weigh more than 3500 kt if made of
aluminum.
A dumbbell shape has the advantage that the radius of curvature of the
part holding the atmosphere can be made small while the radius of
rotation remains large. However, in this configuration people could only
live on the cross section of the spheres, and to hold 10,000 people with
670,000 m^2 of projected area the spheres would have to be 326.5 m in
radius. Together they would weigh about 380 kt.

A torus also permits control of the radius that contains the atmosphere
separately from the radius of rotation. Moreover, the torus can
distribute its habitable area in a large ring. Consequently, the radius
needed to enclose the 670,000 m^2 of projected area can be quite small,
with a correspondingly small mass-about l50 kt for a torus of major
radius 830 m and minor radius 65 m (where the mass of internal structure
is neglected). The advantages of the torus compared to the sphere and
cylinder are discussed further in appendices B and
C which define some criteria and parameters useful for such comparisons. The important point
is that for a given radius of rotation about four times more mass is
required to provide a unit of projected area in a cylinder or a sphere
than in a torus of small aspect ratio. Thus, among the simple, basic
shapes the torus is clearly superior in economy of structural mass.

If minimum structural mass were the only concern, composite structures
would be the choice. Twenty-five pairs of dumbbells would supply 670,000
m^2 with spheres 65 m in radius and a total mass of 72 kt. The spheres
could be made smaller still and formed into a ring to make a beaded
torus. Alternatively, the toruses themselves could be made with quite
small minor radii and either stacked and connected together to form a
kind of banded torus, or built separately to form a group of small,
independent habitats.

However, as pointed out in the previous chapter, it is desirable to
compensate for the artificial and crowded nature of the habitat by
designing it to give a sense of spaciousness. Composite structures are
rejected largely on architectural criteria of environmental perception.
Not only would they be more difficult to build than the simpler shapes,
but also their short lines of sight, little free volume and internal
arrays of closely-spaced cables and supporting members would produce an
oppressive ambience.

If the colony were composed of a number of small structures, there would
be problems of communication and transport between them as well as the
drawbacks of small scale. Nevertheless, as table 4-1 shows, multiple
structures (and composite ones too) offer substantial savings in mass,
and it might well be that some of their undesirable aspects could be
reduced by clever design. It would be an attractive option to be able to
build up a colony gradually out of smaller units rather than to start off
with an initial large scale structure. The subject of multiple and
composite structures is worthy of more consideration.
The various properties of possible configurations are summarized in table 4-1. The parameters show the mass requirements and
indicate the degree of openness of the different structures. The single
torus, although not the best design in many respects, seems to give the
most desirable balance of qualities. Relative to the sphere and cylinder
it is economical in its requirements for structural and atmospheric mass;
relative to the composite structures it offers better esthetic and
architectural properties. Because of its good habitability properties,
large volume, a variety of possible internal arrangements, the
possibility of incremental construction, a clear circulation pattern,
access to zero gravity docks and recreation at the hub, agriculture as an
integral part of the living area, and a clear visual horizon for
orientation, the torus is adopted as the basic form of the habitat. The
dimensions of this single torus are given in the first column of table 4-1.

The need to shield humans adequately from the ionizing radiations of
space imposed some significant design decisions. An ideal shield would
bring the radiation dosage below 0.5 rem/yr cheaply and without impairing
the contact of the colonists with their environment. However, after
considering active shields which electromagnetically trap, repel or
deflect the incident particles, and a passive shield which simply absorbs
the particles in a thick layer of matter, the study group chose the
passive shield for their design.

Active Shields

When a charged particle passes through a magnetic field, its path
curves. Thus, as figure 4-5 shows, the proper configuration of magnetic
field lines can form a shielded region which particles cannot enter.
Since for a given magnetic field the curvature of the path of a particle
is inversely proportional to its momentum, the region is shielded only
against particles below a certain cutoff momentum or cutoff energy.
Particles above this cutoff energy can still penetrate
(ref. 10).

The problems of magnetic shielding become apparent when the cutoff
energy has to be chosen. Protection against heavy ion cosmic rays, the
so-called high-Z primaries (i.e. the iron nuclei and others mentioned in
chapter 2) and most solar flares would be achieved with a cutoff of 0.5
GeV/nucleon. The difficulty is that most secondary particles are created
from the primary flux above 2 GeV/nucleon which can penetrate the shield
and generate secondaries in the mass of the shield itself. As a
consequence a magnetic field around the torus with a cutoff of 0.5
GeV/nucleon and a structural mass of about 10 kt, corresponding to a
thickness of matter of 0.5 t/m^2, would actually increase the exposure to
about 20 rem/yr. Only the addition of shielding to an extent of 1.3 t/m^2
could reduce the dosage to a level equivalent to there being no secondary
particle generation by shielding, that is, about 8 rem/yr. Furthermore,
even then a specially heavily shielded shelter would be required as protection against secondaries produced by the strongest solar flares.
The consequences of the production of secondary particles are shown in
figure 4-6.

A cutoff of 10 or 15 GeV/nucleon would eliminate so many of the high
energy particles that even with secondary production the dose would not
be above 0.5 rem/yr. A shield of this capability would also protect
against the effects of the strongest solar flares, and no shelter would
be needed. The difficulty is that the structural mass required to resist
the magnetic forces between superconducting coils precludes this design
even for the most favorable geometry, namely, a torus.

Similarly, electric shielding by a static charge seems infeasible since
a 10-billion-volt potential would be required for even moderate
shielding. On the other hand, a charged plasma which sustains high
electrical potential in the vicinity of the habitat is a more promising
approach (ref. 11). However, means to develop such a
plasma requires extensive research and technical development before a
charged plasma might be considered for design. Some further details of
this approach are given in appendix D.

Passive Shield

Passive shielding is known to work. The Earth's atmosphere supplies
about 10 t/m^2 of mass shielding and is very effective. Only half this
much is needed to bring the dosage level of cosmic rays down to 0.5
rem/yr. In fact when calculations are made in the context of particular
geometries, it is found that because many of the incident particles pass
through walls at slanting angles a thickness of shield of 4.5 t/m^2 is
sufficient. Consequently it was decided to surround the habitat with this
much mass even though it requires that many millions of tonnes of matter
have to be mined and shipped to the colony.

(Communication), Fraction viewable byinternal line of sightfrom one place

0.02

0.06

0.06

Interior: Openess

Poor

Poor

Poor

InteriorPopulation capacity at 67 m^2/person

10,000

10,000

10,000

Table 4-1 shows the shielding masses required for different
configurations; the single torus requires 9.9 Mt of shield. This much
mass cannot be rotated at the same angular velocity as the habitat
because the resultant structural stresses would exceed the strength of
the materials from which the shield is to be built. Consequently the
shield must be separate from the habitat itself and either rotated with
an angular velocity much less than 1 rpm or not rotated. To minimize the
mass required, the shield would be built as close to the tube of the
torus as possible, and therefore the rotating tube would be moving at 87
m/s (194 mph) past the inner surface of the shield from which it is
separated by only a meter or two. The consensus of the study group was
that the engineering necessary to assure and maintain a
stable alignment between the moving torus and its shield would not, in
principle, be difficult. However, no attention in detail was given to
this problem.

The conservative design criteria presently adopted for permanent life in
space are derived from research on Earth and in space, especially Skylab
missions, that gives very little indication of the actual effects of
living in space for many years. In the time leading up to the
colonization of space more information will become available, and it may
lead to substantial changes in the configuration proposed in this study.

Higher Population Density

A very simple change would be to reduce the amount of area available per
person. Under these circumstances several of the structures described in
table 4-1 would be made less massive. By placing the agriculture outside
the shielded area and by reducing the remaining projected area available
from 47 m^2 per person to 35 m^2, substantial savings could be made in both
structural and shielding mass (table 4-2).

This 25 percent increase in crowding may not be so drastic as it appears, since use can be made of
the three dimensionality of space in a way more effective than is done on
Earth. With sufficiently large overhead spaces between levels, several
levels could be
included in a habitat while maintaining an impression of openness. This
approach would be particularly advantageous if the gravity criteria were
relaxed as well.

Lower Simulated Gravity and Higher Rotation Rates

It is particularly interesting to examine the consequences of
simultaneously relaxing the requirements of pseudogravity and rotation
rate. If instead of 0.95 +/- 0.05 g and 1 rpm, the design allows 0.85 +/-
0.15 g and 1.9 rpm some interesting possibilities emerge. Under these new
conditions, parameters for the same geometries discussed earlier are
summarized in table table 4-2

(Communication), Fraction viewable byinternal line of sightfrom one place

0.01

0.07

0.1

Interior:Openess

Good

Fair

Poor

InteriorPopulation capacity at 35 m^2/person

10,000

10,000

10,000

A major consequence is that the radius of
rotation now becomes 236 m as figure 4-4
confirms.

With this new radius of rotation neither a single torus nor a single
dumbbell can supply sufficient space for a colony of 10,000. A cylinder,
as before, supplies far too much. The sphere, on the other hand, supplies
exactly the right amount and becomes an attractive possibility for a
habitat. As the table shows, however, multiple and composite structures
would still be contenders although they would be even more deficient in
the desirable architectural and organizational features.

To be more specific, figure 4-7 illustrates a possible spherical design
with the agriculture placed in thin toruses outside the shielded sphere.

This configuration has been named the Bernal sphere in honor of J. D.
Bernal (ref. 12). When the Bernal sphere is compared with its nearest
competitor, the banded torus, it is seen to be particularly efficient in
its shielding requirements, needing 300,000 t less than the banded torus
and millions of tonnes less than any other configuration. The Bernal
sphere, however, requires from 3 to 4 times as much atmospheric mass as
the other possible forms, and from 2 to 4 times as much structural mass.

Higher Radiation Exposures

As more is learned about the effects of ionizing radiation, it is
possible that larger exposures to radiation might be found to be
acceptable. Such a change in this criterion would make active magnetic
shielding an interesting possibility and might also favor the development
of a plasma shield. Of course, if higher levels of radiation became
acceptable, a smaller amount of passive shielding would be needed so that
the mass of shielding might become less significant in determining
habitat design.

Any of these changes might shift the favored emphasis from one geometry
to another. A choice of a particular form would again have to balance
aesthetic against economic requirements, and it is certain that more
investigation of this problem will be necessary. A particularly important
question is the relative cost of shielding mass, structural mass, and
atmospheric mass. Knowledge of these costs is basic to deciding which
geometric alternative to select.

Although the construction of large structures in space places strong
emphasis on fabrication techniques, relatively little attention was
devoted to the subject by the summer study group. The few alternatives
considered did not seem to be mutually exclusive, but instead mutually
supportive. Only a brief description of these alternatives is given.

Initial Construction Facilities

Fabrication facilities needed to build the habitat and supporting
factories and power plants were described at a Princeton Conference, May
1975, on metal forming in space by C. Driggers.This proposal has been
adopted. Standard tecnnology for hot and cold working metals is
sufficient to form the sheet, wire and structural members needed. An
extensive machine shop must be provided so that many of the heavy
components of a rolling mill, extrusion presses, casting beds and other
equipment can be made at the space colony rather than have to be brought
from Earth.

Building the Habitat Shell

Assembly of the habitat from aluminum plate and ribs proceeds first from
the spherical hub (including docking facilities) outward through the
spokes to start the torus shell. Both the spokes and shell are suitable
for construction by a "space tunneling" concept in which movable end caps
are gradually advanced along the tube as construction proceeds. This
allows "shirt-sleeve" conditions for workmen as they position
prefabricated pieces brought through the spokes and make the necessary
connection. Large pieces of shield are placed around the completed
portions as the slag material becomes available from the processing
plant. Internal structures are built when convenient. However, every effort must be made to complete
the basic shell and the first layer of shielding as quickly as possible
so that spin-up can begin, gravity can be simulated, and the construction
crew and additional colonists can move in to initiate life support
functions within the habitat. A critical path analysis will reveal the
best sequencing of mirror, power plant, shield, and internal
construction.

An alternative technology for fabrication in space, which deserves more
investigation, is the making of structures by metal-vapor molecular
beams. This is discussed in more detail in appendix E. If proved out in
vacuum chamber experiments, this technique may cut the labor and capital
costs of converting raw alloys into structures by directly using the
vacuum and solar heat available in space. Its simplest application lies
in the fabrication of seamless stressed-skin hulls for colony structures,
but it appears adaptable to the fabrication of hulls with extrusive
window areas and ribs, as well as to rigid sheet-like elements for
zero-gravity structures such as mirrors and solar panels.

A simple system might consist of a solar furnace providing heat to an
evaporation gun, which directs a conical molecular beam at a balloon-like
form. The form is rotated under the beam to gradually build up metal
plate of the desired strength and thickness. While depositing aluminum,
the form must be held at roughly room temperature to ensure the proper
quality of the deposit.

Structures Inside the Habitat

To fulfill the criteria set forth in chapter 2, a lightweight, modular
building system must be developed to serve as an enclosing means for the
various spatial needs of the colony.

Modular building systems developed on Earth can be categorized into
three general types: that is, box systems using room-size modules;
bearing-panel systems; and structural-frame systems. A box system entails
asembling either complete shells or fully completed packages with
integrated mechanical subsystems. Bearing-panel systems use load-bearing
wall elements with mechanical subsystems installed during erection.
Structural-frame systems use modularized framing elements in combination
with nonload-bearing wall panels and mechanical subsystems which are
normally installed during erection. Other systems which have seen limited
application on Earth but would be appropriate in the colony include:
cable supported framing systems with nonload-bearing fabric and panel
space dividers, and pneumatic air structures using aluminum foil and
fiberglass fabrics with rigid, aluminum floor elements.

In selecting a baseline configuration, box systems were rejected because
they normally involve the duplication of walls and floors and tend to be
overly heavy. If metal vapor deposition is developed as a forming
technique however, this type of system would become highly desirable.
Bearing-panel systems were likewise rejected since they do not allow
integration of mechanical subsystems except during erection, and since
walls are heavy because they are load bearing. Cable and pneumatic
systems were rejected due to their inability to span short distances
without special provisions. However, they might be highly desirable
because of their flexibility and lightness if a lower gravity environment
proves acceptable in the colony.

The system that appears most suitable for use in the colony might
involve a light, tubular structural frame (composed of modular column and
beams) in combination with walls that are nonload bearing and with
prepackaged, integrated mechanical subsystems (such as bathrooms) where
needed. This system provides lightweight modularity to a high degree,
good spanning capabilities, easily obtainable structural rigidity, and
short assembly time since all labor intensive mechanical systems are
prefabricated. A schematic (ref. 13) of some possible components of such
a system is shown in figure 4-8. Applications of such a system to the
colony are many and could be applied to all necessary enclosures with
proper adaptation to the various specialized needs of life in space.

Some of the possible materials and components investigated as especially
suitable for building in space are illustrated in
appendix F. Elements that are light and strong and could be made from materials available in
space are favored. The exterior and interior walls and the floor
components are built from these materials. The floor components are based
on extremely light yet strong elements designed for Skylab.

It is not usual to think of human population as something to be
designed. Nevertheless the numbers, composition, age and sex
distribution, and productivity of the colonists bear importantly on the
success of the project and on the creation of a suitable design. The
study had to consider who should be the colonists, how many there should
be, what skills they must have, and how they should organize and govern
themselves. The alternatives are numerous and the grounds for choosing
between them not as definite as for the more concrete problems of
engineering, but it was possible to make what seem to be reasonable
choices based on the goals of having in space permanent communities of sufficient productivity to
sustain themselves economically.

Size and Suitability of Population

It is possible in principle to specify a productive task, for example,
the manufacture of solar power satellites, and then calculate the number
of people necessary to perform it, the number needed to support the
primary workers, and the number of dependents. The sum of such numbers
does not accurately define the population needed to found a colony since
the calculation is complex. Even a casual consideration of what is
necessary for a truly closed society would suggest that a colony
population be far in excess of any reasonable first effort in space.

A similar approach would bypass the calculation just described and
simply copy the population size and distribution of a major productive
urban center on Earth. The difficulty, however, is that such communities
are quite large, on the order of some hundreds of thousands of people.
Moreover, close inspection reveals that human communities on Earth are
less productive by labor force measurement standards than what would be
needed in at least the early stages of space colonization.

One way to have a colony more productive than Earth communities would be
to make the colony a factory, populated only by workers. The colony would then be only a space station, with crews of workers rotated in and out,
much as is done on the Alaska pipeline project. Aside from the serious
problems of transportation, such an approach does not meet the goal of
establishing permanent human communities in space.

In the face of these difficulties a rather arbitrary decision is made to
design for a colony of 10,000 with an attempt to bias the population in
directions that favored high productivity but does not compromise too
badly the goal of setting up a community in which families live and
develop in a normal human way. It is also assumed that the completed
colony is not an isolated single undertaking, but is a first step in a
rapidly developing program to establish many colonies in space.

Ethnic and National Composition

The possible variations in nationality or ethnic composition are in
principle very great. The actual composition will depend largely on who
sponsors and pays for the colonization. If colonization were undertaken
as a joint international project, the composition of the population would
surely reflect that fact. On balance, however, it seems reasonable for
the purposes of this design to assume that the first space colony will be
settled by persons from Western industrialized nations.

Age and Sex Distributions

The initial population of the first colony is projected to grow from a
pool of some 2000 construction workers who, in turn, bring immediate
family members numbering an additional one to three persons per worker.
Selective hiring of construction crew members tends to bias this
population toward certain highly desirable skills, and toward the younger
ages. In anticipation of the labor needs of the colony and the need to
avoid the kinds of burdens represented by large dependent populations, a
population is planned with a smaller proportion of old people, children
and females than the typical U.S. population. It is a close analog of
earlier frontier populations on Earth.

The proposed population is conveniently described in terms of
differences from the population of the United States as described in the
1970 Census (ref. 14). These changes are illustrated in
figure 4-9 which
compares the colony with the composition of a similar sized community on
Earth. The sex ratio is about 10 percent higher in favor of males,
reflecting both the tendency of construction workers to be male and the
expectation that by the time construction begins in space an appreciable
fraction of terrestrial construction workers are female. Partly for this last
reason and partly because of the anticipated need for labor in the colony, sizable increase in the proportion of
married women in the labor force is assumed. Most striking is the
substantial shift of the population out of the more dependent ages - from
under 20 and over 45 into the 21 to 44 age class.

Export Workers

Productivity of any community is importantly influenced not only by the
size of the labor force but also by the share of worker output going for
export. Numbers on modern U.S. communities (see, e.g.,
appendix G) indicate that in our complex society the percentage engaged in export
activity is generally less in the larger cities than in the smaller
towns. The maximum activity for export seems to be about 70 percent.
Without taking into account the peculiarities of life in space, the study
group assumes that 61 percent of the workforce, or nearly 44 percent of
the population of the initial colony would be producing for export (see
fig. 4-9). This percentage declines as the colony grows. Conversely at an
early stage in its development when the population is about 4300, the
workforce is about 3200, with 2000 producing for export.
Appendix G provides the data from which these assumptions are derived.

Social Organization and Governance

The form and development of governance depend strongly on the cultural
and political backgrounds of the first colonists. The subject is rich
with possibilities ranging from speculative utopian innovations to
pragmatic copies of institutions existing on Earth. Among the
alternatives easily envisioned are quasimilitary, authoritarian
hierarchies, communal organizations like kubbutzim, self-organized
popular democracies operating by town meetings, technocratic centralized
control, or bureaucratic management similar to that of contemporary large
corporations.

It seems most likely that government for the initial colony would be
based on types of management familiar in government and industry today.
There would be elements of representative democracy, but the organization
would surely be bureaucratic, especially as long as there is need for
close dependency on Earth. But whatever the forms initially, they must
evolve as the colonists develop a sense of community, and it is easy to
imagine at least two stages of this evolution.

First there is the start of colonization by some Earth-based corporate or
governmental organization. Later, as continued development leads to more
and more settlement, the colonists form associations and create governance
bodies which reflect rising degrees of community identity,
integration and separation of decision making powers from organizations on
Earth. These changes evolve first within a single habitat and then
cooperative and governmental relations develop when neighboring habitats
and a larger community grow. The rate at which this evolution occurs is
uncertain.

What do the colonists eat and how do they obtain this food? What do they
breathe? How do they deal with the industrial and organic wastes of a
human community in space? These questions pose the basic problems to be
solved by life support systems. Richness of life and survival from
unforeseen catastrophes are enhanced by diversification and redundance of
food supplies, energy sources, and systems for environmental control, as
well as by variety of architecture, transportation and living
arrangements, and these considerations are as important in choosing among
alternatives for life support as in making choices among other
subsystems.

Food

Food supplies can be obtained from Earth or grown in space or both.
Total supply from the Earth has the advantage that the colony would then
have no need to build farms and food processing facilities or to devote
any of its scarce labor to agriculture. However, for a population of
10,000 the transport costs of resupply from Earth at 1.67 t/yr per person
is about $7 billion/yr. The preferred choice is nearly complete
production of food in space.

Whatever the mode of production, it must be unusually efficient, thereby
requiring advanced agricultural technologies (ref. 15). Direct synthesis
of necessary nutrients is one possibility, but such biosynthesis is not
yet economically feasible (J. Billingham, NASA/Ames, personal
communication),1. Also, algae culture and consumption have long been
envisioned as appropriate for life in space, but upon close inspection
seem undesirable because algae are not outstandingly productive plants
nor are they attractive to humans (ref. 16). The best choice seems to be
a terrestrial type of agriculture based on plants and meat-bearing
animals (ref. 17).

This form of agriculture has the advantage of depending on a large
variety of plant and animal species with the accompanying improvement in stability of the ecosystem that such diversity
contributes (ref. 18). Moreover, plants and animals can be
chosen to supply a diet familiar to the prospective colonists, that is, a
diet appropriate to a population of North Americans biased in favor of
using those plant and animal species with high food yields.
Photosynthetic agriculture has a further advantage in that it serves as
an important element in regeneration of the habitat's atmosphere by
conversion of carbon dioxide and generation of oxygen. It also provides a
source of pure water from condensation of humidity produced by
transpiration (ref. 19).

Choices of food sources within the general realm of terrestrial
agriculture become a compromise between preference and diversity on the
one hand and efficiency on the other. For the colony, efficient use of
area (even at expense of efficiency measured in other terms, i.e., as
energy) is a critical factor to be balanced against a varied and
interesting diet. For example, the almost exclusive use of rabbits and
goats for animal protein previously proposed (ref. 15) for space colonies
is rejected as being unnecessarily restrictive and seriously lacking in
variety.

Recycling Wastes

High costs of transportation place great emphasis on recycling all the
wastes of the colony. Because in the near future Earth appears to be the
only practical source of elements fundamental to agriculture - carbon,
nitrogen, and hydrogen - they must initially be imported from Earth. To
avoid having to continually import these elements, all wastes and
chemicals are recycled with as small a loss as possible.

Waste water can be treated biologically as in most terrestrial
communities, physiochemically, by dry incineration, or by some more
advanced technique such as electrodialysis, electrolysis, vapor
distillation or reverse osmosis (ref. 20). Each of these alternatives is
ruled out for various reasons. Biological treatment provides only
incomplete oxidation and produces a residual sludge which must then be
disposed of with attendant risks of biological contamination.
Physiochemical treatment has no organic conversion, and is chemically a
difficult process. Dry incineration requires an external energy source to
maintain combustion and it produces atmospheric pollutants. All the
advanced processes are incomplete in that the resulting concentrates
require further treatment.

Wet oxidation (Zimmerman process) has none of the foregoing defects.
Operating at a pressure of 10^7 MPa (1500 lb/in.^2) and a temperature of
260 degrees C, wet oxidation with a total process time of 1-1/2 hr produces a
reactor effluent gas free of nitrogen, sulfur and phosphorous
oxides; a high quality water containing a finely divided
phosphate ash and ammonia. Both the reactor gas and the water are sterile
(refs. 21, 22). At solids concentrations greater than 1.8 percent the
process operates exothermally with an increase in the temperature of the
waste water by 56 degrees C (personal communication from P. Knopp,
Vice-President, Zimpro Processing, Rothschild, Wisconsin). These definite
advantages lead to the choice of this process as the basic technique for
purification and reprocessing within the space colony.

Composition and Control of the Atmosphere

The desired composition of the atmosphere is arrived at as the minimum
pressure needed to meet the criteria for atmospheric safety stated in
chapter 2. This results in the atmospheric composition detailed in
table 4-3.

TABLE 4-3 - HABITAT ATMOSPHERE

T = 20 +/- 5 DEGREES C

Relative humidity = 50 +/- 10 percent

GAS

(kPa)

(mmHg)

O2

22.7

170

N2

26.6

200

CO2

<0.4

<3

Total pressure

50.8

380

Water vapor

1.0

7.5

1 standard atmosphere = 101 kPa

Its outstanding features are: normal terrestrial, partial pressure
of oxygen, partial pressure of carbon dioxide somewhat higher than on
Earth to enhance agricultural productivity, and a partial pressure of
nitrogen about half of that at sea level on Earth. Nitrogen is included
to provide an inert gaseous buffer against combustion and to prevent
certain respiratory problems. Because nitrogen must come from the Earth,
its inclusion in the habitat's atmosphere means there is a substantial
expense in supplying it. This fact, in turn, suggests that it is
desirable to hold down the volume of atmosphere in the habitat, a factor
taken into consideration in the discussion of the habitat geometry given
earlier. The total atmospheric pressure is thus about half that at sea
level on Earth.

Atmospheric oxygen regeneration and carbon dioxide removal are by
photosynthesis using the agricultural parts of the life support system.
Humidity control is achieved by cooling the air below the dewpoint,
condensing the moisture and separating it. Separation of condensate water
in zero gravity areas (such as the manufacturing area and hub) by hydrophobic and hydrophilic materials offers the
advantage of a low pressure drop and lack of moving parts
(ref. 23) and
is the preferred subsystem.

Trace contamination monitoring and control technology is highly
developed due primarily to research done in submarine environments. The
habitat environment is monitored with gas chromatograph mass spectrometer
instruments (ref. 24). Trace contamination control can be effectively
accomplished by absorbtion (e.g., on activated charcoal), catalytic
oxidation, and various inert filtering techniques.

An important goal for the design for space colonization is that it be
commercially productive to an extent that it can attract capital. It is
rather striking then that the study group has been able to envision only
one major economic enterprise sufficiently grand to meet that goal. No
alternative to the manufacture of solar power satellites was conceived,
and although their manufacture is likely to be extremely valuable and
attractive to investors on Earth, it is a definite weakness of the design
to depend entirely on this one particular enterprise. A number of
valuable smaller scale manufactures has already been mentioned in
chapter
2 and, of course, new colonies will be built, but these do not promise to
generate the income necessary to sustain a growing space community.

There is some choice among possible satellite solar power stations
(SSPS). Two major design studies have been made, one by Peter Glaser of
Arthur D. Little, Inc. (ref. 25), and the other by Gordon Woodcock of the
Boeing Aircraft Corporation (ref. 26). Conceptually they are very
similar, differing chiefly in the means of converting solar power to
electricity in space. Woodcock proposes to do this with conventional
turbogenerators operating on a Brayton cycle with helium as the working
fluid; Glaser would use very large arrays of photovoltaic cells to make
the conversion directly.

There is not a great deal to argue for the choice of one system rather
than the other, except perhaps that the turbogenerator technology
proposed by Woodcock is current, while Glaser relies on projections of
present day photovoltaic technology for his designs. In the spirit of
relying on current technology, the Woodcock design seems preferable, but
a definite choice between the two is not necessary at this time. A more
detailed description of the SSPS alternatives with a discussion of
microwave transmission and its possible environmental impact is given in
appendix H.

Chapter 2 surveyed space and described what is there and how space is
shaped in terms of distance, propulsive effort and gravitational
attraction. These aspects of space together with the location of needed
resources are important to choosing a site for the habitat. The community
should be located for convenience with respect to its resources-sunlight,
weightlessness, and minerals - and also with access to and from its
principal market, Earth. The site should be chosen by balancing the needs
of production against the needs of marketing the product.

Near to but not on the Moon

The minerals of space are to be found in the distant outer planets, the
asteroids, the nearer and more accessible planets like Mars, the moons of
other planets, or our own Moon. Of course the Earth is a primary source
of mineral wealth too. It seems reasonable to place the colony near one
of these sources. For reasons explained in the next section, the Moon is
chosen as the principal extraterrestrial source of minerals, hence the
habitat should be near the Moon.

But where should the habitat be placed in the vicinity of the Moon? At
first glance the Moon's surface seems a good choice, but any part of that
surface receives the full force of the Sun's radiation only a small
fraction of the time. Moreover, on the Moon there is no choice of
gravity; it is one-sixth that of Earth and can only be increased with
difficulty and never reduced. Space offers both full sunshine and zero
gravity or any other value of simulated gravity one might choose to
generate. An additional difficulty with a lunar location is related to
the major product of the colonies, SSPS's. Transporting them from the
Moon to geosynchronous orbit is not economically viable. For ease of
exploitation of the properties of space, the habitat should be located in
free space.

In Free Space at L5

Although there is no stable location at a fixed point in space in the
Earth-Moon system, the colony could be located in any one of a number of
orbits in free space. These orbits can be around the Earth, or the Moon,
or both the Earth and the Moon. Those near either the Earth or the Moon
are rejected because of the frequency and duration of solar eclipses
which deprive the colony of its light and energy. Large orbits around the
Earth make it difficult to deliver the large mass of material needed from the Moon, while large orbits around the Moon become orbits in
the Earth-Moon system about which little is known at the present time.
These last two options, while not chosen, present interesting
alternatives which should be examined more closely.

There remain the orbits about the five libration points. Three of these,
L1, L2, and L3, are known to be unstable, and to maintain orbits around
any of these three points for long periods of time requires appreciable
expenditures of mass and energy for station keeping.

There do exist, however, large orbits around both of the remaining
Vibration points, L4 and L5. These have been shown to be stable
(refs.
27, 28). A colony in either of these orbits would be reasonably accessible
from both Earth and Moon. One of these libration points, L5, is chosen for
the location of the first space colony. This choice is somewhat arbitrary
for the differences between L4 and L5 are very slight.

From where will come 10 million tonnes of matter needed to build a
colony? And where and how will it be processed, refined and shaped into
the metals, glass and other necessary structural material? The topography
of space shapes the answer to the first question; human ingenuity offers
answers to the second. A major problem only partly solved is how to
transport large quantities of
matter from mines on the Moon to space. Some possible solutions to that
problem are suggested.

Sources

As noted previously, lunar materials have been chosen to supply the
great bulk of mass necessary for the first colony, including the shell
and internal structure, passive shield, soil, and oxygen. As indicated in
figure 4-10, only a small percentage of the mass, including initial
structures, machinery, special equipment, atmospheric gases other than
oxygen, biomass, and hydrogen for water, comes from Earth.

This decision has been made for a variety of reasons. Of the bodies in
the solar system which might supply materials, the other planets are
eliminated by the expense of transportation from their surfaces, and the
moons of the outer planets by transport times of years and by costs. This
leaves the asteroids, comets, and the moons of Mars.

While the composition of the moons of Mars is unknown, both the comets
and asteroids are apparently abundant sources of organic materials in
addition to rock and possibly nitrogen and free metals as well. For
immediate future applications, however, the Moon's position makes it
attractive and, compared to the asteroids, the Moon has advantages of
known properties, a distance suitable for easy communication, and it
allows perhaps simpler overall logistics.

However, when the space colonization program is begun, technical and
economic imperatives seem likely to drive it quickly toward exploitation
of asteroidal rather than lunar materials and toward much less dependence
on Earth. Long before the results of mining activity on the Moon became
visible from the Earth, the colony program would be obtaining its
materials from the asteroids. Given that source, the "limits of growth"
are practically limitless: the total quantity of materials within only a
few known large asteroids is enough to permit building space colonies
with a total land area many thousands of times that of the Earth.

Processing: Where?

A variety of alternatives exist for the processing of lunar ores to
yield materials for the colony. These involve various combinations of
processing site, materials to be produced, and chemistry. Optimization
requires a detailed analysis of manifold possibilities. The study limited
itself to choosing a plan which seems achievable and advantageous based
on reasonable extrapolations of current technology.

The decision as to whether to process at the colony or on the Moon is
dictated by various factors. The lunar site has the advantage of being
close to the ore source and having a gravity which might be used in some
chemical processing. Lunar processing might be expected to decrease the
amount of material to be shipped to the colony. However, closer
examination reveals that the colony's shielding requirements exceed the
slag production of the processing plant; hence, no transportation is
saved by processing at a lunar site. Moreover, lunar processing also
possesses certain definite disadvantages when compared to processing at
the site of the colony. Plant facilities shipped from the Earth to the
Moon require much greater transportation expense than for shipment to the
colony site. In addition, solar furnaces and power plants are limited to
a 50 percent duty cycle on the Moon. Without power storage this would
curtail operations at a lunar processing site. Radiators for process
cooling are less efficient and, therefore, larger when placed on the
Moon, because they have a view of the Sun or of the hot lunar surface.
Finally, even at only 1/6 of Earth's gravity, components of the plant
have significant weight. On the Moon this requires support structure and
cranes and hoists during assembly. But these are not needed if processing
is done at the colony site. Based on these considerations, it appears
that major processing should take place at the colony site.

Processing: What and How?

The colony requires various materials which are obtainable from the
lunar soil. Silica is needed for windows and solar cells. Oxygen is the
major component of the colony atmosphere and is required for
manufacturing water. It is also a rocket propellant. Silica and oxygen
are essential to the success of the colony and therefore must be
extracted from lunar ore. However, there is some latitude for choice and
optimization among the variety of metals available. Aluminum, titanium,
magnesium and iron are all potential construction materials. Although
aluminum is chosen as our basic structural material, a decision to refine
titanium might have some special advantages. On the Moon, titanium is in
the form of a magnetic mineral (ilmenite) which can, in theory, be easily
separated from the bulk of the lunar ore. In addition, use of titanium
for structure would result in significant savings in the total amount of
refined material because, although more difficult to form and fabricate,
its strength-to-mass ratio is greater than that of the other metals
available. Since ilmenite is basically FeTi03, significant amounts of
iron and oxygen can be extracted as byproducts.

These facts support a recommendation that the alternative of titanium
refining should be studied in detail. Possible methods for refining
titanium are presented in figure 4-11
and discussed in appendix I.

Most of the remaining metal oxides in the ore must be separated from one
another by rather complex techniques before further refining of the
metals. Aluminum is the only other metal which justifies detailed
consideration. In addition to excellent structural properties and
workability it has good thermal and electrical properties (see appendix
A). It is chosen as the principal structural material only because
information concerning titanium processing is somewhat less definite and,
in particular, the magnetic separation technique for lunar ilmenite has
not yet been demonstrated.

The various methods by which aluminum might be refined from lunar
anorthosite are shown schematically in figure 4-12. The system chosen is
melt-quench-leach production of alumina followed by high temperature
electro-winning of aluminum from aluminum chloride. Alternative paths are
discussed in appendix I.

To provide window areas for the space structure, glass is to be
manufactured from lunar materials. Silica (SiO2), the basic ingredient in
glassmaking, is found in abundance on the Moon. However, another basic
constituent, sodium oxide (Na20), which is used in the most common flat
plate and sheet glass industrially produced, is found in only small percentages in the lunar soil. Glass
processing on Earth uses Na2O primarily to lower the melting temperature
that has to be generated by the furnace (refs. 29, 30). Since the solar
furnace to be provided for processing the lunar material will be capable
of generating temperatures considerably higher than those which could
possibly be needed for this process, it appears unnecessary to supply
additional Na20 from the Earth (personal communication, J. Blummer,
Vice-President for Research, Libbey OwensFord Company, Toledo, Ohio, Aug.
1975).

To date, glasses made from lunar soil samples returned by the Apollo
missions have been dark in color. The techniques necessary to manufacture
glass from lunar materials which possesses the properties needed for
efficient transmission of sunlight into a space habitat have not been
demonstrated (personal communication, Pittsburgh Plate Glass Company,
Pennsylvania, Aug. 1975). However, it is believed that additional
materials research will permit glass of adequate quality for a space
facility to be processed from the lunar soil with a minimum of additives
(if any) brought from the Earth (personal communication, D. R. Ulrich,
Air Force Office of Scientific Research, Washington, D. C., Aug. 1975).

A possible technique which may prove feasible in space for large scale
production is the removing of almost all nonsilicate ingredients by leaching
with acid. Again, the availability of high furnace temperatures is a prerequisite to meet the
melting temperature of silica, and the manufacturing process will have to
be shown to be manageable in space. The resulting glass, of almost pure
silica (> 95 percent SiO2), possesses the desirable properties of low
thermal expansion, high service temperature, good chemical, electrical,
and dielectric resistance, and transparency to a wide range of
wavelengths in the electromagnetic spectrum.

Requirements for volume, mass, and energy of a glass-processing unit, a
description of a sample process, and an elaboration of lunar soil
constituents are given in appendix J.

Transport of Lunar Material

The construction of the colony depends critically on the capability of
transporting great quantities of lunar material from the Moon to the
colony without large expenditures of propellant. There are three parts to
this problem: launching the material from the Moon, collecting it in
space, and moving it to the colony. Two principal ways to launch have
been devised, along with some variations.

One method is to launch large payloads, of about 60 t, by firing them from a large gas gun. The gun is operated by using
nuclear power to compress hydrogen gas and then permitting the gas to
expand the length of the launch tube. Because hydrogen must be obtained
from Earth, its replacement is expensive, and consequently after each
launch the gas is recovered through perforations in the end section of
the launch tube which is encased in an enclosed tube. Further details are
given in appendix K.

The system is of interest because of its conceptual simplicity and light
weight. But the principal drawback of the gas gun system is the
difficulty of collecting the payloads once they have been launched
because their dispersion is large. Collection needs a fleet of automated
interceptor rockets. The propellant requirement for interception is about
1 percent of the total mass launched. In terms of technology that may be
available in the near future, these interceptor rockets have to use
chemical propulsion with hydrogen as fuel. The second drawback is that
the gas gun requires the development of sliding seals able to withstand
high pressures and yet move at high velocities and still maintain
acceptable leakage rates. Despite the uncertainties about precision of
aim, the difficulties of automated rendezvous and interception, and the
associated propulsion requirements, the concept appears fundamentally
feasible and worthy of more study. However, the uncertainties are
sufficient to make another alternative more attractive at this time.

The alternative method, which is the one chosen for this design,
involves an electromagnetic mass accelerator. Small payloads are
accelerated in a special bucket containing super conducting coil magnets.
Buckets containing tens of kilograms of compacted lunar material are
magnetically levitated and accelerated at 30 g by a linear, synchronous
electric motor. Each load is precisely directed by damping the vibrations
of the bucket with dashpot shock absorbers, by passing the bucket along
an accurately aligned section of the track and by making magnetic
corrections based on measurements using a laser to track the bucket with
great precision during a final draft period. Alignment and precision are
the great problems of this design since in order to make efficient
collection possible, the final velocity must be controlled to better than
l0^-3 m/s. Moreover, the system must launch from 1 to 5 buckets per second
at a steady rate over long periods of time, so the requirements for
reliability are great. This system is considerably more massive than the
gas gun. More details about it are given in the next chapter.

The problem of catching the material launched by the electromagnetic
mass driver is also difficult. Three possible ways to intercept and gather the stream of material were devised.
Two so-called passive catchers (described in more detail in appendix L),
involve stationary targets which intercept and hold the incoming
material. The other is an active device which tracks the incoming
material with radar and moves to catch it. The momentum conveyed to the
catcher by the incident stream of matter is also balanced out by ejecting
a small fraction of the collected material in the same direction as, but
faster than, the oncoming stream.

An arrangement of catching nets tied to cables running through
motor-driven wheels permits rapid placement of the catcher anywhere
within a square kilometer. By using a perimeter acquisition radar system,
the active catcher tracks and moves to intercept payloads over a
considerably larger area than the passive catchers. Unfortunately this
concept, described in more detail in the next chapter, has the defects of
great mechanical complexity. Nevertheless, although many questions of
detail remain unanswered and the design problems appear substantial, the
active catcher is chosen as the principal means of collecting the
material from the mass launcher on the Moon.

Despite possible advantages it seems desirable not to place the catcher
at the site of the colony at L5. For three reasons L2 is chosen as the
point to which material is launched from the Moon.

First, the stream of payloads present an obvious hazard to navigation,
posing the danger of damage if any of the payloads strike a colony or a
spacecraft. This danger is particularly acute in view of the extensive
spacecraft traffic to be expected in the vicinity of the colony. The
payloads, like meteoroids, may well be difficult to detect. Hence, it
appears desirable to direct the stream of payloads to a target located
far from the colony.

Second, L2 is one-seventh the distance of L5 permitting use of either a
smaller catcher or a less accurate mass-driver.

Third, to shoot to L5 requires that the mass-driver be on the lunar
farside. For launch to L2, the mass-driver must be on the nearside. By
contrast, a nearside location for the mass-driver permits use of our
knowledge of Moon rocks brought back in Apollo flights, and there are a
number of smooth plains suitable for a mass-launcher. The nearside also
permits line-of-sight communications to Earth.

Catching lunar material at L2 means that transport must then be provided
to L5. It appears most practical to use mechanical pellet ejectors
powered by an onboard nuclear system of 25 MW. This same system is used
to offset the momentum brought to the catcher by the payloads arriving
at up to 200 m/s.

The transportation requirements of a colony are much more extensive than
merely getting material cheaply from the Moon to the factories of the
colony. There must be a capability for launching about 1 million tonnes
from the Earth over a total period of 6 to 10 years. There must be
vehicles capable of traversing the large distances from Earth to L5 and
to the Moon. There must be spacecraft that can land equipment and people
on the Moon and supply the mining base there. Fortunately, this is a
subject to which NASA and the aerospace industry have given considerable
thought; the study group relied heavily on this work. A schematic
representation of the baseline transportation system is shown in
figure 4-13.

From Earth's Surface to Low Orbit

The space shuttle is to be the principal U.S. launch vehicle for the
1980s. However for space colonization applications, the shuttle has low
payload per launch and requires too many flights with excessive launch
costs per kg. At the other end of the launch vehicle spectrum, a number
of advanced concepts have been studied. These include a large winged
"Super-Shuttle," fully-reusable ballistic transporters resembling giant
Mercury capsules, and even use of a laser rocket with a remote energy
source. Such concepts are not considered in this primary study because of
uncertain technologies, excessive development costs, and long leadtimes.
However, one concept for the "F-l flyback" is discussed in
appendix C of chapter 6.

The colony has to rely on lift vehicles derived from and, therefore,
dependent on the shuttle and other already-developed boosters. Studies
have been made on shuttle-derived heavy lift launch vehicles with two and
with four solid boosters (fig. 4-14). In these, the manned shuttle
vehicle is replaced with a simple vehicle having automated avionics and
increased freight capability. The four-booster configuration has a
payload of 150 t at under $20 million per launch.

A discussion of the environmental impact on the ozone layer of Earth by
launch vehicles is given in appendix N.

Transport Beyond Low Earth Orbit

For routine transport of people and freight, the system uses
single-engine vehicles employing space-storable, liquid-gas propellants
in modular tankage. The NERVA
nuclear rocket is rejected in favor of the space shuttle main engine
(SSME). NERVA offers high performance but represents a new development,
and involves the safety considerations associated with nuclear systems.
The SSME represents an available, well-understood engine. Moreover, with
oxygen for refueling available at L5 from processing of lunar ores in
industrial operations, the SSME vehicle performance would approach that
of NERVA. Consequently the SSME as shown in
figure 4-15 has been
selected. Details are given in appendix M.

For passenger transport, the launch vehicle cargo fairing accommodates a
passenger cabin holding 200 people. A single SSME could also be used to
land over 900 t of cargo on the lunar surface.

For transport of major systems involving their own large power plants,
electric propulsion is feasible. Such systems include the L5 construction
shack with its 300 MW power plant, and the solar-power satellites to be
built at the colony for delivery to geosynchronous orbit. Candidate
propulsion systems include ion rockets, resistojets, and mechanical
pellet accelerators. In particular, for the baseline system, large
numbers of standard ion thrusters are clustered, thus permitting
application of current electric-propulsion technology. It is possible in
the future that a Kaufman electrostatic thruster could be developed with
oxygen as propellant. As described in the next chapter, a rotary pellet
launcher is proposed to power the tug which brings the lunar ore from L2
to the processing plant at L5.

Thus the system described in chapter 1 is
arrived at. It carries 10,000
colonists in a toroidal habitat positioned at L5 orbiting the Sun in
fixed relation to the Earth and Moon and exploiting the paths through
space in figure 4-16.
Mining the Moon for oxygen, aluminum, silica, and
the undifferentiated matter necessary for shielding, the colonists ship a
million tonnes per year by electromagnetic mass launcher to L2. There,
with the active catcher, the material is gathered and transshipped to L5
to be refined and processed. With small amounts of special materials,
plastics, and organics from Earth, the colonists build and assemble solar
power stations which they deliver to geosynchronous orbit. The colonists
also raise their own food and work on the construction of the next
colony. The following chapter gives a more detailed picture of how the
various parts work together.